In the field of the optical network serving as the base in the Internet communication network and the like, the optical MEMS (Micro Electro Mechanical System) technique attracts attention as a technique that realizes a multichannel, wavelength division multiplex (WDM), low-cost system, and an optical switch has been developed using this technique (for example, see Japanese Patent Laid-Open No. 2003-57575). One of the most characteristic constituent components of the MEMS type optical switch is a mirror array. The optical switch enables line switching without converting light into an electrical signal or demultiplexing multiplexed light by wavelength division demultiplex. When a trouble occurs in a line in use, the optical switch is used to distribute the signal to another line, so that the communication state is maintained.
In the mirror array, a plurality of mirrors are arranged two-dimensionally to form a matrix. The mirror array comprises a mirror substrate and an electrode substrate arranged to oppose it. The mirror substrate has a plurality of movable structures serving as mirrors, and a support member which supports the movable structures rotatably by spring members such as torsion springs. The electrode substrate is obtained by forming a plurality of electrode portions corresponding to the movable structures serving as the mirrors on a substrate serving as the base.
FIGS. 28 and 29 show an example of a conventional mirror device. A mirror device 700 has a structure in which a mirror substrate 800 having a mirror and an electrode substrate 900 having electrodes are disposed parallel to each other.
The mirror substrate 800 has a plate-like frame portion 810 having an opening which is almost circular when seen from the top, a gimbal 820 having an opening which is almost circular when seen from the top and disposed in the opening of the frame portion 810 by a pair of gimbal connectors 811a and 811b, and a mirror 830 which is disposed in the opening of the gimbal 820 by a pair of mirror connectors 821a and 821b and is almost circular when seen from the top. The frame portion 810, gimbal connectors 811a and 811b, gimbal 820, mirror connectors 821a and 821b, and mirror 830 are integrally formed of, e.g., single-crystal silicon. A frame-like member 840 surrounding the gimbal 820 and mirror 830 is formed on the upper surface of the frame portion 810. The frame-like member 840 is fixed to the frame portion 810 through an insulating layer 850.
The pair of gimbal connectors 811a and 811b respectively comprise torsion springs and are formed in the notches of the frame portion 810 to connect the frame portion 810 to the gimbal 820. The gimbal 820 can rotate about a gimbal rotation axis x extending through the pair of gimbal connectors 811a and 811b in FIG. 28.
Similarly, the pair of the mirror connectors 821a and 821b respectively are formed in the notches of the gimbal 820 and comprise torsion springs to connect the gimbal 820 to the mirror 830. The mirror 830 can rotate about a mirror rotation axis y extending through the pair of mirror connectors 821a and 821b in FIG. 28. The gimbal rotation axis x and mirror rotation axis y are orthogonal. Consequently, the mirror 830 rotates about two orthogonal axes.
The electrode substrate 900 has a plate-like base 910, and a terraced projection 920 projecting from the surface (upper surface) of the base 910 and formed at a position to oppose the mirror 830 of the opposing mirror substrate 800. The base 910 and projection 920 are made of, e.g., single-crystal silicon. The projection 920 comprises a prismoidal second terrace 922 formed on the upper surface of the base 910, a prismoidal first terrace 921 formed on the upper surface of the second terrace 922, and a columnar pivot 930 formed on the upper surface of the first terrace 921. The pivot 930 is formed almost at the center of the first terrace 921. Thus, the pivot 930 is disposed at a position opposing the center of the mirror 830.
Four electrodes 940a to 940d are formed on the four corners of the projection 920 and the upper surface of the base 910 continuous to the four corners to fall within a circle concentric with the mirror 830 of the opposing mirror substrate 800. A pair of protrusions 960a and 960b which line up to sandwich the projection 920 are formed on the upper surface of the base 910. Furthermore, interconnections 970 are formed between the projection 920 and protrusion 960a and between the projection 920 and protrusion 960b on the upper surface of the base 910. The interconnections 970 are connected to the electrodes 940a to 940d through lines 941a to 941b. 
The mirror substrate 800 and electrode substrate 900 as described above form a mirror device 700 as shown in FIG. 29 as the lower surface of the frame portion 810 is bonded to the upper surfaces of the protrusions 960a and 960b such that the mirror 830 opposes the electrodes 940a to 940d that oppose it.
In the mirror device 700, the mirror 830 is grounded, and positive voltages are applied to the electrodes 940a to 940d while forming asymmetric potential differences among the electrodes 940a to 940d, so that the mirror 830 can be attracted by an electrostatic attracting force and rotated in an arbitrary direction.
In the mirror device 700, when applying driving voltages to the electrodes 940a to 940d, the pivot 930 prevents the mirror 830 from being entirely attracted by the electrodes 940a to 940d to be parallel to them to collide against them. The pivot 930 also serves as a fulcrum about which the mirror 830 rotates.